JP2004312042A - Manufacturing method for flexible printed circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a flexible printed circuit board which controls curling to the utmost, has superior dimensional stability, and can fully cope with macro circuits. <P>SOLUTION: A metal composite film is manufactured by forming a first polyimide precursor layer by coating varnish for the first polyimide precursor, containing a polyamic acid component obtained by the reaction of acid anhydride and an amine component and a polyimide component, obtained by reaction of the acid anhydride and an isocyanate component in a range of 10 % to 70 % on a conductor 2, and by forming a second polyimide precursor layer coating varnish for the second polyimide precursor consisting of the polyamic acid component, obtained by reaction of the acid anhydride and the amine component on the first polyimide precursor layer. Then, a first polyimide resin layer 3a and a second polyimide resin layer 3b are formed by imidizing the first polyimide precursor layer and the second polyimide precursor layer by applying the metal composite film to heat treatment. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a flexible printed circuit board, which is a base substrate of a flexible printed circuit board used for electrical connection of electronic devices and the like, and more particularly, to a flexible printed circuit board in which an insulator made of a polyimide resin layer is formed on a conductor. A method for producing the same.

  In a so-called portable electric product such as a portable recording / reproducing device, in order to reduce the size and the cost, the electric circuit units are connected using a flexible printed wiring board which is relatively inexpensive and can save space. There are many things. In a conventional flexible printed wiring board, for example, a circuit is formed by etching a conductor of a flexible printed board as a base substrate.

  As this flexible printed circuit board, a polyimide film is frequently used in terms of flexibility and heat resistance. Specifically, as shown in FIG. 8, a varnish for a polyimide precursor made of polyamic acid was directly applied on a copper foil 51, and the polyimide precursor was dried and then imidized to form a polyimide resin layer 52. A two-layer flexible printed circuit board 50 has been proposed and put into practical use.

  However, the two-layer flexible printed circuit board 50 has a problem that curling occurs because the polyamic acid applied on the metal foil 51 is imidized at a high temperature.

  One of the causes of such curling is due to the difference in the coefficient of thermal expansion between the metal foil 51 and the polyimide resin layer 52. This is considered to be due to the difference in the heat shrinkage rate from that of No. 52.

  Therefore, in order to remove the curl in a two-layer flexible printed circuit board, a method is proposed in which a polyimide resin layer formed on a metal foil has a multilayer structure to mitigate the difference in coefficient of linear thermal expansion between the metal foil and the polyimide resin layer. Proposed in reference 1. In this method, it is known that curl can be removed quite effectively when a flexible printed circuit board is completed by forming a polyimide resin layer on a copper foil.

  In the above-mentioned method, since the coefficient of linear thermal expansion of the polyimide resin layer is adjusted in a state including the copper foil, the copper foil is etched to form a circuit pattern after the completion of the flexible printed circuit board, thereby producing a flexible printed circuit board. In some cases, curling occurs again. In other words, this method has a problem in that it is impossible to correct the curl of the film after the circuit is formed.

  Another cause of the curl of the two-layer flexible printed circuit board is considered to be that the material itself shrinks when the polyimide precursor is imidized, and the shrinkage causes curl.

  At present, effective measures have not been taken against such a phenomenon that curl occurs due to material contraction when the polyimide precursor is imidized. For this reason, it is difficult to completely remove the curl in the conventional two-layer flexible printed circuit board, and the curl causes a decrease in the precision of the conductor spacing of the circuit after etching. It is not suitable for forming a finer circuit pattern.

  Further, when such a polyimide precursor is imidized, material shrinkage occurs, so that there is a problem that distortion occurs between the copper foil 51 and the polyimide resin layer 52 to deteriorate the adhesive strength.

  Further, in the conventional two-layer flexible printed circuit board, when actually imidizing, as shown in FIG. 9, a film 53 in which a varnish for a polyimide precursor made of polyamic acid is applied on a copper foil is wound into a roll. In this state, heat treatment is performed to perform imidization.

  As described above, since the heat treatment is performed in the roll state at the time of imidization, the heat treatment cannot be uniformly performed on the entire rolled film 53, and as a result, the flexible printed circuit board 50 that has been unrolled after imidization cannot be wound up. In each of the portions corresponding to the core 53a, the intermediate portion 53b, and the outer portion 53c closest to the shaft, the adhesive strength between the copper foil 51 and the polyimide resin layer 52 varies, so that uniform quality cannot be obtained. is there. Specifically, especially in a portion corresponding to the intermediate portion 53b from the core portion 53a, the adhesive strength between the copper foil and the polyimide resin layer was not stable.

JP-A-8-250860

  SUMMARY OF THE INVENTION The present invention has been proposed in view of the above-described situation, and has improved adhesive strength, and furthermore, uniformity of quality with reduced variation in adhesive strength, and curl as small as possible. It is an object of the present invention to provide a method capable of manufacturing a flexible printed circuit board which can sufficiently cope with a fine circuit having excellent stability.

  In the method for producing a flexible printed board according to the present invention, a polyamic acid component represented by the following chemical formula 3, in which an acid anhydride component and an amine component are reacted on a conductor, and an acid anhydride component and an isocyanate component: Forming a first polyimide precursor layer by applying a varnish for a first polyimide precursor containing the polyimide component represented by the following chemical formula 4 in a range of 10% to 70%, On the first polyimide precursor layer, a second polyimide precursor varnish composed of a polyamic acid component formed by reacting an acid anhydride component and an amine component is applied to form a second polyimide precursor layer. Forming a metal composite film, and performing a heat treatment on the metal composite film to imidize the first polyimide precursor layer and the second polyimide precursor layer, thereby forming a first polyimide precursor layer and a second polyimide precursor layer. And forming a polyimide resin layer and the second polyimide resin layer.

  As described above, in the method for manufacturing a flexible printed circuit board according to the present invention, in addition to the polyamic acid component represented by Chemical Formula 3, as the polyimide precursor that is the material of the first polyimide resin layer in contact with the conductor, A polyimide precursor layer is formed using one containing the indicated polyimide component.

  That is, in the method for manufacturing a flexible printed circuit board according to the present invention, as the polyimide precursor that is the material of the first polyimide resin layer, a material that has been partially imidized in advance before imidization is used. The material shrinkage due to the imidization of the polyimide precursor, which is the material of the polyimide resin layer 1, is suppressed. As a result, according to the method for manufacturing a flexible printed circuit board according to the present invention, the distortion generated between the conductor and the first polyimide resin layer during imidization is suppressed, and the conductor and the first polyimide resin layer are not bonded to each other. The adhesive strength is improved.

  Moreover, in the method for manufacturing a flexible printed board according to the present invention, the above-described material shrinkage is suppressed to a small value when imidizing by rolling, so that the core portion, the intermediate portion, and the In each portion corresponding to the winding portion, the variation in the adhesive strength between the conductor and the first polyimide resin layer is suppressed, and the quality is made as uniform as possible.

  Furthermore, in the method for manufacturing a flexible printed circuit board according to the present invention, curl is suppressed as much as possible because material shrinkage due to imidization of the polyimide precursor is suppressed.

  Hereinafter, a method for manufacturing a flexible printed circuit board according to the present invention will be described in detail with reference to the drawings.

  Prior to the description of the present invention, a flexible printed circuit board manufactured by the method of the present invention will be described.

  As shown in FIG. 1, the flexible printed board 1 manufactured by the method of the present invention has a first polyimide resin layer 3a and a second polyimide resin layer 3b sequentially laminated on one main surface 2a of a copper foil 2. It becomes. The flexible printed board 1 has a desired circuit pattern formed on the copper foil 2 by etching to form a flexible printed wiring board used for electrical connection of electronic devices and the like.

  As the copper foil 2, specifically, an electrolytic copper foil, a rolled copper foil, or the like can be used. The thickness of the copper foil 2 is preferably 35 μm or less, and more preferably 8 μm to 18 μm in order to form a fine circuit. Here, if the thickness of the copper foil 2 is 18 μm or more, it is difficult to form a fine circuit. If the thickness of the copper foil 2 is 8 μm or less, wrinkles and the like are likely to occur in the coating process, making it difficult to work.

In addition, the copper foil 2 has a property that the coefficient of linear thermal expansion increases when the copper foil 2 is heat-treated in an atmosphere of 250 ° C. to 400 ° C., which is the imidization temperature of the first polyimide resin layer 3a and the second polyimide resin layer 3b. For example, linear thermal expansion coefficient of the copper foil 2 is before imidization is ^{16.0 × 10 -6 /K~18.0×10 -6 / K} , 18.0 × 10 -6 comes after imidation / K to 20.0 × 10 ⁻⁶ / K.

  The copper foil 2 is optimally a copper foil that is not subjected to a surface treatment. However, when the copper foil 2 is subjected to a surface treatment with zinc, chromium, oxidation, or the like, the center line average roughness Ra is 10 μm or less, preferably 7 μm or less. .

  The conductor used for the flexible printed board 1 according to the present invention is not limited to the copper foil 2 but may be a metal foil such as aluminum or iron. Further, a metal foil made of an alloy of these metals and beryllium, nickel, chromium, tungsten, or the like, for example, a beryllium copper foil or a stainless steel foil, or a composite foil of copper and aluminum may be used.

  Then, the surface of these metal foils may be subjected to mat treatment, nickel or zinc plating, oxidation treatment, or the like in order to improve the adhesive strength. It is also possible to perform a chemical surface treatment such as an aluminum alcoholate, an aluminum chelate, a silane coupling agent, and an imidazole treatment.

  In particular, in the flexible printed circuit board 1 of the present invention, the first polyimide resin layer 3a in contact with the copper foil 2 is composed of a polyamic acid component represented by Chemical Formula 5 obtained by reacting an acid anhydride component and an amine component, and an acid anhydride component. It is formed by imidizing a polyimide precursor containing a polyimide component represented by Chemical Formula 6 obtained by reacting a product component and an isocyanate component.

言い換えれば、本発明のフレキシブルプリント基板１では、第１のポリイミド樹脂層３ａの材料として用いる第１のポリイミド前駆体用ワニスが、化６に示すようなポリイミド成分を含有することにより、イミド化工程前に既に部分的にイミド化された状態である。詳しくは、後述するように、第１のポリイミド前駆体用ワニスは、ジイソシアネート成分により部分イミド化されている。

In other words, in the flexible printed circuit board 1 of the present invention, the varnish for the first polyimide precursor used as the material of the first polyimide resin layer 3a contains a polyimide component as shown in Chemical formula 6, whereby the imidization step is performed. It has already been partially imidized before. Specifically, as described later, the first varnish for polyimide precursor is partially imidized by a diisocyanate component.

  The first polyimide resin layer 3a is formed through an imidization process after the first varnish for polyimide precursor is applied on the copper foil 2 and dried.

The component shown in Chemical formula 5 is a general polyamic acid component, and is obtained by condensing an acid dianhydride and an aromatic diamine. Therefore, Ar ¹ and Ar ² shown in Chemical Formula 5 are aromatic ring portions composed of a condensate of an acid dianhydride and an aromatic diamine shown below. The aromatic ring portion may be appropriately substituted.

  Examples of the acid dianhydride include pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, bis (dicarboxyphenyl) sulfonic dianhydride, bis (dicarboxyphenyl) ether dianhydride, and benzophenonetetracarboxylic acid An acid dianhydride or the like can be used.

  Examples of the aromatic diamine include o-, m- and p-phenylenediamine, 4,4 diaminodiphenyl ether, 2,2-bis [4- (4-aminophenoxy) phenyl] propane, and 2,2-bis [4 -(4-aminophenoxy) phenyl] sulfone, 4,4-diaminobenzanilide, 1,4-bis (4-aminophenoxy) benzene, 3,3′-dimethyl-4,4′-diaminobiphenyl and the like are preferred. Can be used.

The component shown in Chemical formula 6 is obtained by reacting the acid dianhydride with an aromatic diisocyanate (imidation reaction) to form an imide ring. Therefore, Ar ³ and Ar ⁴ shown in Chemical Formula 6 are aromatic ring portions composed of a condensate of the following aromatic diisocyanate compound and the above-mentioned acid dianhydride. The aromatic ring portion may be appropriately substituted.

  Examples of the aromatic diisocyanate compound include 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, tolidine diisocyanate, 1,5-naphthalene diisocyanate, hexamethylene diisocyanate, and xylylene diisocyanate.

  As described above, in the flexible printed circuit board 1 manufactured by the method of the present invention, the first varnish for the polyimide precursor used as the material of the first polyimide resin layer 3a in contact with the copper foil 2 is composed of the polyimide component represented by the above chemical formula 6. It contains.

  That is, in the flexible printed circuit board 1 manufactured by the method of the present invention, the polyimide precursor, which is the material of the first polyimide resin layer 3a, is partially imidized in advance before imidization, and thus the first The material shrinkage due to the imidization of the polyimide precursor, which is the material of the polyimide resin layer 3a, can be suppressed. As a result, according to the flexible printed board 1 of the present invention, the distortion generated between the copper foil 2 and the first polyimide resin layer 3a during imidization is suppressed, and the copper foil 2 and the first polyimide resin layer The adhesive strength with 3a can be improved.

  Moreover, in the flexible printed circuit board 1 manufactured by the method of the present invention, the above-mentioned material shrinkage is suppressed to a small value when the film is rolled and imidized. In each part corresponding to the part and the winding part, variation in the adhesive strength between the copper foil 2 and the first polyimide resin layer 3a is suppressed, and the quality can be made as uniform as possible and high quality.

  Furthermore, in the flexible printed circuit board 1 manufactured by the method of the present invention, since the material shrinkage due to the imidization of the polyimide precursor is suppressed to a small extent, the curl is suppressed as much as possible and good flatness is obtained. .

At this time, in the first polyimide precursor varnish used for the first polyimide resin layer 3a, the proportion of the component shown in Chemical Formula 6 is 100 mol of the sum of the component shown in Chemical Formula 5 and the component shown in Chemical Formula 6. %, It is preferably from 10 mol% to 70 mol%, more preferably from 30 mol% to 50 mol%.

  Here, the number of moles of the aromatic diisocyanate compound used is defined as an imidization ratio of the polyamic acid. That is, the varnish for the first polyimide precursor of the first polyimide resin layer 3a contains 10% by mole to 70% by mole of imidization before the imidization step by containing the polyimide component as shown in Chemical formula 6. And more preferably 30 to 50 mol% imidization. In other words, the first varnish for a polyimide precursor has a partially imidized ratio (hereinafter referred to as a charged imidization ratio) of preferably 10 mol% to 70 mol%, and more preferably 30 mol%. More preferably, it is 50 mol%.

When the charged imidization ratio is less than 10 mol%, the flexible printed circuit board 1 is apt to be curled. On the other hand, when the imidation ratio is 70 mol% or more, the adhesive strength between the copper foil 2 and the first polyimide resin layer 3a decreases. In addition, a polyimide resin having a coefficient of linear thermal expansion of 30 × 10 ⁻⁶ / K or less is a polar solvent such as N-methyl-2-pyrrolidone or dimethylacetamide when a part of the precursor polyamic acid is imidized. And the resin is separated or the polyamic acid solution is gelled or has a high viscosity. In such a state, it is difficult to apply on the copper foil 2. Therefore, the upper limit of the charged imidization ratio is limited to 70 mol%.

  Further, the first polyimide precursor varnish used for the first polyimide resin layer 3a is subjected to a heat treatment at a high temperature by a subsequent imidization treatment, so that thermal expansion due to the high-temperature heat treatment is reduced. It is necessary to use a material having a high glass transition temperature Tg within the above range of the imidation ratio. Ideally, the glass transition temperature Tg of the first polyimide precursor varnish is preferably 250 ° C. or higher.

  Here, when the temperature is 250 ° C. or less, the imidization temperature requires a high temperature of 300 ° C. to 400 ° C., and thus the adhesive strength of the first polyimide resin layer 3a to the copper foil 2 in the width direction and the longitudinal direction, When a flexible printed wiring board in which a circuit is formed by etching and removing the copper foil 2 of the flexible printed board 1 is manufactured, a large variation is likely to occur in the shrinkage ratio of the flexible printed wiring board, resulting in poor dimensional stability. .

  On the other hand, the second polyimide resin layer 3b is coated with a second polyimide precursor varnish mainly composed of a polyamic acid which is a condensation compound of an acid dianhydride and an aromatic diamine as a precursor of the polyimide resin. After being dried and imidized, it is formed.

  As the acid dianhydride and the aromatic diamine, those similar to the acid dianhydride and the aromatic diamine used for the polyimide precursor of the first polyimide resin layer 3a as described above can be used.

  Regarding the polyamic acid solution, which is a polyimide precursor of any of the first and second polyimide resin layers 3a and 3b, the carboxylic acid group of the polyamic acid may corrode a metal foil such as the copper foil 2. Therefore, a rust preventive can be added. Moreover, the rust preventive can also improve the adhesive strength to the metal foil in addition to the rust preventive function. Examples of such a rust preventive include a triazole compound such as 3- (N-salicyloyl) amino-1,2,3-triazole and an imidazole compound such as 2-methylimidazole and a salt thereof. The amount of such a rust inhibitor is preferably 1 to 10 parts by weight based on 100 parts by weight of the polyamic acid.

  Further, in the polyimide resin layer having a multilayer structure, in order to improve the adhesive strength so as not to cause delamination between the individual polyimide resin layers 3a and 3b, or to improve the adhesive strength with a metal foil such as the copper foil 2. An epoxy resin may be added for improvement. As this epoxy resin, for example, a general-purpose epoxy resin such as a bisphenol type or a novolak phenol type can be used. The epoxy resin curing agent is not always necessary, but may be added. In that case, the curing agent may be blended in the polyamic acid solution.

The second polyimide resin layer 3b preferably has a coefficient of linear thermal expansion of less than 30 × 10 ⁻⁶ / K. That is, the polyimide precursor for the second polyimide resin layer 3b preferably has a coefficient of linear thermal expansion after imidization of less than 30 × 10 −6 / K. If the coefficient of linear thermal expansion of the second polyimide resin layer 3b is 30 × 10 −6 / K or more, when the copper foil 2 of the flexible printed board 1 is removed by etching to produce a flexible printed wiring board, This is because the printed wiring board does not become flat.

  As described above, in the flexible printed board 1 of the present invention, the curl of linear thermal expansion of the second polyimide resin layer 3b laminated on the first polyimide resin layer 3a in contact with the copper foil 2 is defined, thereby curling. Can be more effectively suppressed, and the formation of a fine circuit required for further high-density mounting can be realized.

  Here, in order to set such a coefficient of linear thermal expansion, JP-A-60-157286, JP-A-60-243120, JP-A-63-239998, JP-A-1-245586, As reported in JP-A-3-123093 and the like, the coefficient of linear thermal expansion can be freely adjusted by changing the combination of existing acid dianhydrides and aromatic diamines, their respective chemical structures, and their mixing ratios. Can be adjusted.

  The flexible printed circuit board to which the present invention is applied is not limited to a structure including two polyimide resin layers as shown in FIG. 1, but has a multilayer structure in which a plurality of polyimide resin layers are laminated on a copper foil 2. Is also good. At this time, it is particularly preferable that the polyimide resin layer has a multilayer structure of three or less layers. This is because if the number of layers is four or more, cost increases and it is not economical.

  Specifically, as shown in FIG. 2, as the flexible printed circuit board 20 of the present invention, a first polyimide resin layer 13a, a second polyimide resin layer 13b, and a third polyimide resin layer 13c are formed on the copper foil 2. The layers are sequentially formed. The first polyimide resin layer 13a is formed by imidizing the partially imidized first polyimide precursor varnish, similarly to the first polyimide resin layer 3a in the flexible printed circuit board 1 shown in FIG. It becomes. The second polyimide resin layer 13b is formed by imidizing a conventionally known polyamic acid solution similarly to the second polyimide resin layer 3b in the flexible printed circuit board 1 shown in FIG.

  Here, similarly to the second polyimide resin layer 13b, the third polyimide resin layer 13c is formed by imidizing a conventionally known polyamic acid solution. In the flexible printed circuit board 20, the third polyimide resin layer 13c located on the outside preferably has a higher coefficient of linear thermal expansion than the second polyimide resin layer 13b located at the center. With such a configuration, the adhesive strength with the copper foil 2 can be improved, and curling of the flexible print base 20 can be suppressed.

  In particular, it is more preferable that the coefficient of linear thermal expansion of the first polyimide resin layer 13a on the copper foil 2 side is slightly larger than the coefficient of linear thermal expansion of the outer third polyimide resin layer 13c. This is because the surface roughness of the copper foil 2 affects the curl.

  As described above, by forming the polyimide resin layer into a multilayer structure, the coefficient of linear thermal expansion can be adjusted to a value closer to the coefficient of linear thermal expansion of the copper foil 2, and curling can be suppressed. In particular, curl can be controlled by the thickness of the polyimide resin in the outer layer with respect to the copper foil 2.

  When the purpose of the flexible printed board of the present invention is only to improve the adhesive strength to the copper foil 2 and to suppress the variation in the adhesive strength, the polyimide resin layer formed on the copper foil 2 May be a single layer, and the polyimide precursor may be imidized using a partially imidized polyimide precursor as shown in Chemical Formulas 5 and 6 to form a polyimide resin layer. However, as described above, in order to more effectively realize not only the adhesive strength but also curl suppression, the heat ray of the second polyimide resin layer laminated on the first polyimide resin layer in contact with the copper foil 2 is required. More preferably, the expansion coefficient is defined.

  Next, a manufacturing method according to the present invention for manufacturing the flexible printed circuit board 1 configured as described above will be described with reference to the drawings.

  First, a copper foil 2 which is a conductor as shown in FIG. 3 is prepared.

  Next, a varnish for a first polyimide precursor, which is a material of the first polyimide resin layer 3a formed on the copper foil 2, is synthesized and adjusted as follows. First, an excess acid dianhydride and an aromatic diamine are dissolved in a solvent and reacted to produce a polyamic acid prepolymer having acid dianhydrides at both terminals. Then, an aromatic diisocyanate compound is added to the polyamic acid prepolymer, and the acid dianhydride in the polyamic acid prepolymer is reacted with the aromatic diisocyanate compound to directly produce a polyamic acid solution having an imide ring. At this time, the reaction with the aromatic diisocyanate compound can be performed under mild conditions such as 60 ° C., for example, since the acid dianhydride reacts more easily than the carboxyl group of the polyamic acid.

  As the solvent, for example, a pyrrolidone-based solvent such as N-methyl-2-pyrrolidone, an acetamide-based solvent such as N, N'-dimethylacetamide, and a phenol-based solvent such as cresol can be used. In view of the above, use of N-methyl-2-pyrrolidone is preferred. In addition, xylene, toluene, ethylene glycol monoethyl ether and the like can be used as a mixture.

  As described above, the partially imidized first polyimide precursor varnish for the first polyimide resin layer 3a is synthesized.

  Next, as shown in FIG. 4, the first varnish for polyimide precursor synthesized as described above is coated with a known coating method such as a knife coater or a bar coater on the copper foil 2 to have a film thickness of 1 μm to 5 μm. And then dried in a continuous drying oven to evaporate a predetermined amount of the solvent to form a first polyimide precursor layer 31 which has already been partially imidized. The continuous drying furnace is preferably an arch furnace or a floating furnace in order to suppress the occurrence of curling, but is not limited thereto.

  Here, the drying temperature is determined based on the remaining volatilization amount% of the polyimide precursor layer. This residual volatilization amount is considered to be the sum of the undried solvent and the condensed water from imidization.

  At this time, the residual volatilization amount of the first polyimide precursor layer 31 is preferably 20% to 30%. If it is 20% or less, the adhesive strength with the second polyimide resin layer 3b to be formed later decreases, and delamination tends to occur. On the other hand, if it is 30% or more, the adhesive strength between the metal foil and the polyimide resin layer in the completed flexible printed circuit board 1 and the shrinkage of the flexible printed circuit board 1 after imidization are reduced in the core portion, the intermediate portion and the winding portion. This is because they are not stable and cannot be of uniform quality. If it is 30% or more, foaming may occur during imidization.

  Next, a second varnish for polyimide precursor for the second polyimide resin layer 3b formed on the polyimide precursor layer 31 is prepared as follows.

  The second varnish for a polyimide precursor is synthesized by reacting the above-described acid dianhydride with an aromatic diamine in the same polar solvent as the varnish for the first polyimide precursor. Since this reaction is an exothermic reaction, the reaction is controlled while cooling as necessary. Usually, the reaction is carried out at about 0 ° C to 90 ° C, preferably at about 5 ° C to 50 ° C. When the viscosity of the solution is high, the viscosity can be reduced by heat treatment at a temperature close to 90 ° C. At this time, the acid dianhydride and the aromatic diamine may be added at the same time, or one of them may be dissolved or suspended in a polar solvent first, and the other may be reacted while being gradually added. Good. The molar ratio of the acid dianhydride to the aromatic diamine is desirably equimolar, but one of the two components may be used in an excess amount in the range of about 10: 9 to 9:10.

  As described above, the varnish for the second polyimide precursor used for the second polyimide resin layer is synthesized.

  Next, the varnish for the second polyimide precursor synthesized in this way is applied on the first polyimide precursor layer 31, as shown in FIG. 5, and then dried in a continuous drying oven to remove the solvent. A predetermined amount is volatilized to form the second polyimide precursor layer 32, and the metal composite film 30 is obtained. As the continuous drying furnace, the same one as in the case of forming the first polyimide precursor layer 31 can be used.

  At this time, it is preferable to set the residual volatilization amount of the first polyimide precursor layer 31 and the second polyimide precursor layer 32 together to 30% to 50%. If it is 30% or less, the adhesive strength to the second polyimide resin layer 3b formed by imidization later will be reduced, and interlayer delamination is likely to occur. If it is 50% or more, foaming occurs in the next step. Therefore, the temperature of the continuous drying oven is not particularly limited as long as foaming due to scattering of the solvent does not occur. For example, when N-methyl-2-pyrrolidone is used as the solvent, its boiling point is 204 ° C. Should be set to 170 ° C. in order to reduce the temperature to 50% or less.

  Next, in the first polyimide precursor layer 31 and the second polyimide precursor layer 32 formed through such steps, a considerable amount of solvent remains in each precursor layer. It is preferable that the solvent is volatilized by performing a heat treatment.

  Usually, the drying temperature at this time is preferably from 210 ° C. to 250 ° C., and it is preferable to adjust the remaining volatile amount of the entire polyimide precursor layers 31 and 32 to about 7% to 10% by this heat treatment. When the temperature is 250 ° C. or higher, the polyamic acid in the first polyimide precursor layer 31 in contact with the copper foil 2 is imidized, and the imidation ratio may be out of the above-described predetermined range before the imidization treatment. . As a result, it is difficult to suppress the curl of the metal composite film 30. At 210 ° C. or lower, the residual volatilization cannot be reduced to 7% or lower. If the residual volatilization amount is not reduced to 7% or less by this heat treatment, blocking occurs at the time of imidization in a roll shape in the next step, and the metal composite films 30 adhere to each other.

  Next, as shown in FIG. 6, the metal composite film 30 is wound into a roll and subjected to a heat treatment in a temperature range of 300 ° C. to 350 ° C. to perform the first and second pomide precursor layers 31. , 32 are imidized at a stretch to finally produce a flexible printed circuit board 1 having first and second polyimide resin layers 3a, 3b formed on a copper foil 2 as shown in FIG.

  In addition, in order to produce a flexible printed board having three or more polyimide resin layers, a polyimide precursor layer is sequentially applied and dried to form a laminate in the same manner as in the above-described step of FIG. The imidization may be performed in the same manner as described above.

  Further, as an example of a conventionally proposed flexible printed circuit board, there is one in which a polyimide resin layer formed on a conductor has a structure as shown in Chemical formula 6. However, this polyimide resin layer has a structure as shown in the above Chemical Formula 6 by imidizing a polyimide precursor composed of an acid anhydride and a diamine.

  On the other hand, in the flexible printed circuit board 1 manufactured by the method of the present invention, as described above, the first polyimide resin layer 3a is formed by using a polyimide precursor containing a component shown in Chemical Formula 6, which has already been partially imidized. The polyimide precursor is formed by completely imidizing a non-imidated component of the polyimide precursor.

  Therefore, the above-described conventional example and the present invention are different from the conventional example in the constituent components of the polyimide precursor of the polyimide resin layer formed on the conductor.

  Another example of a conventionally proposed flexible printed circuit board is one in which a multilayered polyimide resin layer is formed on a conductor. This flexible printed circuit board forms a multilayer polyimide precursor layer by repeatedly applying and drying the polyimide precursor after coating and drying the polyimide precursor on the conductor. Thereafter, the polyimide precursor layer having a multilayer structure is imidized at a stretch to form a polyimide resin layer having a multilayer structure.

  Here, every time the polyimide precursor is applied and dried, the polyimide precursor is slightly imidized. Therefore, at a stage before the multilayer polyimide precursor layer is finally imidized at once, the multilayer polyimide precursor is already partially imidized.

  However, in this conventional flexible printed circuit board, although it is partially imidized, it is very difficult in production to control the imidation rate of this partial imidization, and when mass production is taken into consideration, uniform quality is obtained. It is very disadvantageous in manufacturing things.

  On the other hand, in the flexible printed circuit board 1 manufactured by the method of the present invention, the polyimide precursor layer in contact with the conductor is partially imidized before the final imidization, which is apparent from the above-mentioned conventional example. Seems identical. However, in the above conventional example, the polyimide precursor is slightly imidized in the drying step after coating the polyimide precursor, whereas the polyimide precursor used in the present invention is an isocyanate from the beginning. The polyimide precursor is partially imidized by containing a component, and such a polyimide precursor which has been partially imidized in advance is positively used.

  In other words, the above-mentioned conventional example and the present invention seem to be the same in that they are slightly imidized before completely imidating the polyimide precursor layer having a multilayer structure, but the method for partially imidizing is completely different. . In addition, in the present invention, since the imidization is partially performed by the isocyanate component, the imidization rate at the time of partial imidization, that is, the charged imidization rate can be easily controlled. Is advantageous.

  Hereinafter, examples of the present invention will be described based on specific experimental results.

  First, a polyamic acid was synthesized as follows. Then, in order to measure the linear thermal expansion coefficient of a polyimide resin obtained by imidizing the synthesized polyamic acid solution, the following polyimide film was produced.

<Synthesis of partially imidized polyamic acid solution>
Synthesis example (1) -1
First, 120 g (0.6 mol) of 4,4′-diaminodiphenyl ether (DPE) is dissolved in 2.0 kg of N-methyl-2-pyrrolidone, and while maintaining the temperature at 20 ° C., pyromellitic dianhydride is then dissolved. A product (PMDA) (218 g, 1.0 mol) was added and reacted for 1 hour to obtain a prepolymer having acid anhydrides at both ends.

  Next, 105.6 g (0.4 mol) of tolidine diisocyanate (TODI) was added to the acid anhydride prepolymer, and the temperature was gradually raised to 60 ° C. At the beginning of the reaction, bubbles of carbon dioxide were generated and the viscosity increased. Then, the mixture was reacted for 2 hours while dispensing 2.0 kg of N-methyl-2-pyrrolidone to obtain a partially imidized polyamic acid solution.

Synthesis Example (1) -2 to Synthesis Example (1) -9
As shown in Table 1, partially imidized polyamic acid was prepared in the same manner as in Synthesis Example (1) -1 except that the types and proportions of the acid dianhydride, aromatic diamine, and aromatic diisocyanate were changed. A solution was obtained.

  The properties of each partially imidized polyamic acid solution obtained as described above were evaluated as follows. Table 1 shows the results.

(1) Measurement of Charged Imidation Rate The proportion of the partially imidized polyamic acid solution that had already been imidized was taken as the charge imidation rate, and the charge imidation rate of this polyamic acid solution was measured as follows.

Using an infrared absorption spectrometer, the absorption amount of the imide group at an absorption wavelength of 1780 cm ^-1 was calculated from the percentage of the absorption amount of the imide group when the sample was imidized at 100 mol% by the surface reflection method (ATR method). .

(2) Measurement of Viscosity and Solid Content The viscosity and the fixed content of this partially imidized polyamic acid solution were measured by a B-type viscometer and loss on drying at 300 ° C., respectively.

(3) Measurement of Glass Transition Temperature The partially imidized polyamic acid solution was applied to a 35 μm-thick electrolytic copper foil, the solvent was dried, and then imidization was performed at 350 ° C. for 10 minutes. Thereafter, the copper foil was removed by etching to obtain a polyimide film. The glass transition temperature of the polyimide film thus obtained was measured by a differential scanning calorimeter method (DSC method).

<Synthesis of polyamic acid solution>
Synthesis Example (2) -1
First, 0.866 kg (8.00 mol) of paraphenylenediamine (PDA, manufactured by Mitsui Chemicals, Inc.) and 4,4′-diaminodiphenyl ether (DPE, Wakayama Seiki) were placed in a 60-liter reactor equipped with a jacket capable of controlling the temperature. 1.603 kg (8.00 mol) of Kasei Co., Ltd. was dissolved in about 44 kg of a solvent N-methyl-pyrrolidone (NMP, manufactured by Mitsubishi Chemical Corporation) under a nitrogen gas atmosphere. Thereafter, the mixture was reacted at 50 ° C. for 3 hours while gradually adding 3.523 kg (16.14 mol) of pyromellitic dianhydride (PMDA, manufactured by Mitsubishi Gas Chemical Company). Thus, a polyamic acid solution having a solid content of about 12% and a viscosity of 20 Pa · S at 25 ° C. was obtained.

  Next, this polyamic acid solution was applied on a copper foil so that the thickness of the film after imidization was 25 μm, and the solvent was scattered in a continuous furnace at 80 ° C. to 160 ° C., and then 230 ° C. to 350 ° C. And heated at 350 ° C. for 30 minutes for imidization.

  Thereafter, the copper foil was removed by etching with a cupric chloride solution to prepare a polyimide film.

Synthesis Example (2) -2 to Synthesis Example (2) -7
As shown in Table 2, a polyamic acid solution was synthesized in the same manner as in Synthesis Example (2) -1, except that the types and proportions of the acid dianhydride and the aromatic diamine were changed.

  The coefficient of linear thermal expansion of the polyimide film obtained as described above was measured as follows, and the results are shown in Table 2.

(4) Measurement of coefficient of linear thermal expansion For the obtained polyimide film, a thermal mechanical analyzer (TMA: SCC150CU, manufactured by S11) was used to apply a load of 2.5 to 5.0 g to a tensile method at 100 ° C. to 350 ° C. The coefficient of linear thermal expansion was measured according to the measurement data in the temperature range of ° C.

  Next, a flexible printed circuit board was manufactured using the polyamic acid solution obtained as described above.

<Production of flexible printed circuit board>
Example 1
First, an electrolytic copper foil (trade name: CF-T9-LP, manufactured by Fukuda Metals) having a thickness of 18 μm and a width of 540 μm was prepared, and the partially imidized synthetic example (1) -1 was formed on this copper foil. The polyamic acid solution was continuously applied so as to have a thickness after drying of 2 μm, and dried to form a first polyimide precursor layer. At this time, the remaining volatile amount was 25%.

  Next, the polyamic acid solution of Synthesis Example (2) -1 was continuously applied on the first polyimide precursor layer so that the thickness after imidization became 22 μm, and dried. A precursor layer was formed. At this time, the residual volatilization amount of the first and second polyimide precursor layers was 30%.

  Further, on this, the polyamic acid solution of Synthesis Example (2) -2 was continuously applied so as to have a thickness of 3 μm after imidization, and dried to form a third polyimide precursor layer. At this time, the remaining volatilization amount of the three layers of the first, second and third polyimide precursor layers was 38%.

  Next, the metal composite film in which the first, second, and third polyimide precursor layers were sequentially laminated on the copper foil 2 was subjected to a heat treatment in a continuous drying oven at 230 ° C. to evaporate excess solvent. At this time, the residual volatilization amount of the three layers was 7.9%.

  Next, 100 m of the metal composite film was wound around a 250 mm diameter stainless steel tube with the copper foil inside. Then, the roll-shaped metal composite film was placed in a batch oven capable of nitrogen replacement, and nitrogen gas was blown into the oven to perform nitrogen replacement until the oxygen concentration became 0.1%. Thereafter, the temperature was raised while blowing nitrogen gas to reach 350 ° C. over about 1 hour.

  Thereafter, the temperature was maintained at 350 ° C. for 15 minutes to completely imidize the first, second, and third polyimide precursors, thereby forming first, second, and third polyimide resin layers. Thereafter, the temperature was lowered to 200 ° C. in a nitrogen atmosphere and cooled in the air.

  Then, the imidized film was unwound while being wound up in a roll shape, and finally, a long flexible printed board was obtained.

Examples 2 to 14 and Comparative Example 1
As shown in Table 3, a flexible printed circuit board was obtained in the same manner as in Example 1 except that the type, thickness, and residual volatilization amount of the polyimide resin layer were changed.

  The following evaluation tests were performed on the flexible printed circuit boards obtained as described above, and the measurement results are shown in Tables 3 and 4.

Method of Evaluation Test (5) Measurement of Residual Volatility Each time each polyimide precursor layer was coated and dried, it was expressed as a percentage of the total volatile components contained in the formed metal composite film.

  That is, the weight of the dried metal composite film is w0, the weight of the flexible printed board formed by imidization is w1, and the weight of the composite polyimide film obtained by removing the metal foil from the imidized flexible printed board by etching. Is defined as w2, the residual volatilization amount is represented by the following equation (1).

Residual volatilization amount = (w0−w1) × 100 / {w0− (w1−w2)} (1)
Therefore, the above w0 was measured each time the polyimide precursor layer was formed, and the w1 and w2 were measured by imidating the polyimide precursor layer. Then, the measured values of w0, w1, and w2 were substituted into the above equation (1) to determine the residual volatilization amount.

(6) Measurement of Adhesive Strength The flexible printed circuit board of each example was rewound into a roll, and the outside of the winding, the intermediate portion and the core located at about 50 m in the longitudinal direction were sampled.

  Then, the copper foil of each portion is formed by etching to a width of 1.59 mm for each of the winding outside, the intermediate portion, and the core portion, and the copper foil is peeled off in accordance with JIS C6471. The adhesive strength was measured by peeling off the surface in the vertical direction at 90 degrees.

(7) Measurement of dimensional stability (shrinkage ratio) Dimensional stability of single-sided copper-clad laminate A method according to JIS C6471 was followed. Specifically, a mark having a size of 210 mm × 210 mm is attached to each part of the outside, the middle part, and the core of the flexible printed circuit board of each embodiment, and the distance between the mark is measured. Thereafter, the copper foil was removed by etching to produce a composite polyimide film, the composite polyimide film was dried, and the distance between the etched and dried marks was measured. The ratio was calculated to determine the dimensional stability. Furthermore, the composite polyimide film was left at 280 ° C. for 10 minutes, and the dimensional stability after the heat treatment was measured in the same manner.

(8) Measurement of curl A flexible printed circuit board was cut into a size of 100 mm x 100 mm to prepare a test piece. The test piece was allowed to stand at room temperature for a predetermined time, and then placed on a horizontal plate in a convex state. The radius of curvature was calculated from the average of the heights of the four corners when mounted. After the test piece was left at 260 ° C. for a predetermined time, the radius of curvature of the curl was calculated in the same manner.

  Moreover, the copper foil of the flexible printed circuit board of the example was removed by etching to prepare a composite polyimide film, and the curvature radius of the curl after leaving each composite polyimide film at room temperature and 260 ° C. in the same manner as described above. Was calculated.

  As shown from the results of Table 4 above, Examples 1 to 14 in which the first polyimide resin layer was formed using the first polyimide precursor varnish partially imidized with diisocyanate, The adhesive strength with the copper foil is very excellent, and the variation in the adhesive strength at the outside of the winding, the middle part and the core part is small. In addition, in Examples 1 to 14, it was found that the shrinkage ratio, that is, the dimensional stability variation was small in the outside of the winding, the intermediate portion, and the core.

On the other hand, Comparative Example 1, which did not use the polyimide precursor varnish partially imidized with diisocyanate, had poor adhesion strength with the copper foil, and had a large variation in adhesion strength at the outside of the winding, the middle part, and the core. . Moreover, in Comparative Example 1, the shrinkage rate itself was large, the dimensional stability was poor, and the variation in the shrinkage rate was large at the outside of the winding, in the middle part, and at the core.

Further, it was found that the curls of Examples 1 to 14 were suppressed to be smaller than that of Comparative Example 1.

  From the above results, the first polyimide resin layer in contact with the copper foil is formed by imidizing the polyimide precursor that has already been partially imidized with diisocyanate, and in particular, the adhesive strength with the copper foil is improved. In addition, it has been found that variations in the adhesive strength at the outside of the winding, the intermediate portion, and the winding core are reduced, the quality is made as uniform as possible, and the dimensional stability is improved. In addition, it has been found that curling can be suppressed.

  In Example 14, in which the charged imidation ratio of the polyimide precursor as the material of the first polyimide resin layer was 80 mol%, the varnish for the polyimide precursor could not actually be applied. From this, as shown in Examples 1 to 13, it is preferable that the charge imidation ratio of the polyamic acid solution, which is the material of the first polyimide resin layer, is 10 mol% to 70 mol%. found.

  Furthermore, Examples 1 to 11 in which the charged imidization ratio is 30 mol% to 50 mol% are more excellent in the adhesive strength than Examples 12 and 13 in which the charged imidization ratio is out of the above range. And the curl is smaller. From this, it was found that the charging imidation ratio of the polyamic acid solution as the material of the first polyimide resin layer was more preferably 30 mol% to 50 mol%.

Further, the ninth, tenth, and eleventh embodiments in which the second polyimide resin layer formed on the first polyimide resin layer has a coefficient of linear thermal expansion of 30 × 10 ⁻⁶ / K or more, The curl is larger as compared with Examples 1 to 7 in which the polyimide resin layer of No. 3 has a coefficient of linear thermal expansion of 30 × 10 ⁻⁶ / K or less. From this, it is understood that when the coefficient of linear thermal expansion of the second polyimide resin layer formed on the first polyimide resin layer is 30 × 10 ⁻⁶ / K or more, the effect of suppressing curling is reduced. Therefore, it was found that the coefficient of linear thermal expansion of the second polyimide resin layer formed on the first polyimide resin layer was more preferably 30 × 10 ⁻⁶ / K or less from the viewpoint of curling suppression.

  In the method for manufacturing a flexible printed circuit board according to the present invention, the polyimide precursor which is a material of the first polyimide resin layer in contact with the conductor uses a material which has been partially imidized in advance before imidization. The material shrinkage due to the imidation of the polyimide precursor, which is the material of the polyimide resin layer, can be suppressed.

  As a result, according to the method for manufacturing a flexible printed circuit board according to the present invention, the bonding strength between the conductor and the first polyimide resin layer is improved, and the variation in the bonding strength is suppressed, so that the quality is made as uniform as possible. At the same time, it is possible to provide a high-quality flexible printed circuit board having excellent flatness by suppressing curling as much as possible.

1 is a cross-sectional view of an example of a flexible printed circuit board to which the present invention has been applied. It is sectional drawing of another example of the flexible printed circuit board to which this invention is applied. It is sectional drawing which shows the process of preparing a copper foil in the manufacturing process of the flexible printed circuit board to which this invention is applied. FIG. 3 is a cross-sectional view showing a step of forming a first polyimide precursor layer on a copper foil in a flexible printed circuit board manufacturing process to which the present invention is applied. FIG. 4 is a cross-sectional view showing a step of forming a second polyimide precursor layer on a first polyimide precursor layer in a manufacturing process of a flexible printed circuit board to which the present invention is applied. It is a perspective view which shows the process of performing a imidation process by making into a roll in the manufacturing process of the flexible printed circuit board to which this invention is applied. FIG. 4 is a cross-sectional view showing a process in which a roll is released after imidization and a flexible printed board is obtained in a flexible printed board manufacturing process to which the present invention is applied. FIG. 11 is a cross-sectional view illustrating an example of a conventional flexible printed circuit board. It is a perspective view which shows the imidation process at the time of producing the conventional flexible printed circuit board.

Explanation of reference numerals

1,20 flexible printed circuit board, 2 copper foil, 3a, 13a, first polyimide resin layer, 3b, 13b second polyimide resin layer, 13c third polyimide resin layer

Claims (4)

A polyamic acid component represented by the following chemical formula 1 formed by reacting an acid anhydride component and an amine component on a conductor, and a polyimide represented by the following formula 2 formed by reacting an acid anhydride component and an isocyanate component. A step of applying a varnish for a first polyimide precursor containing the components in a range of 10% to 70% to form a first polyimide precursor layer;
On the first polyimide precursor layer, a second polyimide precursor varnish composed of a polyamic acid component formed by reacting an acid anhydride component and an amine component is applied to form a second polyimide precursor layer. Forming and forming a metal composite film;
Performing a heat treatment on the metal composite film to imidize the first polyimide precursor layer and the second polyimide precursor layer to form a first polyimide resin layer and a second polyimide resin layer. A method for manufacturing a flexible printed circuit board, comprising:

The method for producing a flexible printed circuit board according to claim 1, wherein the second varnish for a polyimide precursor has a coefficient of linear thermal expansion after imidization of 30 ^10-6 / K or less.   The second polyimide precursor layer having a plurality of layers is formed by repeatedly applying and drying the varnish for the second polyimide precursor on the first polyimide precursor layer. Item 7. A method for manufacturing a flexible printed circuit board according to Item 1. Before imidizing the first polyimide precursor layer and the second polyimide precursor layer, the first polyimide precursor layer and the second polyimide precursor layer are heated at a predetermined temperature that does not promote imidization of the first polyimide precursor layer. 2. The method according to claim 1, further comprising the step of volatilizing a predetermined amount of the solvent from the polyimide precursor layer.

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